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Vol. 8. Issue 5.
Pages 4079-4093 (September - October 2019)
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Vol. 8. Issue 5.
Pages 4079-4093 (September - October 2019)
Original article
DOI: 10.1016/j.jmrt.2019.07.017
Open Access
Photocatalytic degradation of anti-inflammatory drug using POPD/Sb2O3 organic-inorganic nanohybrid under solar light
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Jannatun Zia, Mohammed Rashad P, Ufana Riaz
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Uriaz@jmi.ac.in

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Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi 110025, India
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Tables (3)
Table 1. IR absorption bands of POPD/Sb2O3 nanohybrids.
Table 2. Apparent second-order rate constants (k) of IB degradation and linear regression coefficients from a plot of 1/C versus time.
Table 3. Comparison of reported photocatalytic efficiencies of Sb2O3.
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Abstract

The present manuscript reports the degradation of a non-steroidal anti-inflammatory drug (NSAID) Ibuprofen (IB) in aqueous solution using Sb2O3 and POPD/Sb2O3nanohybrids using solar light radiation. The prepared POPD/Sb2O3 nanohybrids were characterized by TGA, FE-SEM-EDX, FT-IR, XRD, TEM, and UV–Vis studies. The thermal stability was found to follow the order: 24%-POPD/ Sb2O3 > 18%-POPD/Sb2O3 > 12%-POPD/Sb2O3 > 6%-POPD/Sb2O3 > Sb2O3. Infrared Spectroscopy (IR) studies confirmed synergistic interaction between POPD and Sb2O3. High-resolution transmission electron microscopy (HR-TEM) studies revealed the formation of nano cubes of Sb2O3 with particles size ranging between 6 nm–50 nm. X-ray diffraction (XRD) showed crystalline morphology. UV–Vis diffuse reflectance spectroscopy (DRS) studies showed that the band gap of Sb2O3, POPD, and POPD/Sb2O3 nanohybrids were found to be 3.35 eV, 1.57 eV and 1.67 to 1.35 eV respectively. Ibuprofen drug was chosen as a model reaction to evaluate the photocatalytic activities of Sb2O3, POPD and POPD/Sb2O3 using solar light radiation. The catalytic activities of pure Sb2O3 and nanohybrids were in the order of 24%-POPD/Sb2O3 > 18%-POPD/Sb2O3 > 12%-POPD/Sb2O3 > 6%- POPD/Sb2O3 > Sb2O3.The nanohybrids were observed to degrade the drug pollutant by 91% within a short span of 60 min. The degradation kinetics fitted the second order model. Radical scavenging experiments validated that the photo-generated OH and O2– radicals were the two main photoactive species which were responsible for photocatalytic degradation. The degraded drug fragments were identified using liquid chromatography-mass spectrometry (LCMS). The possible photocatalytic mechanism has been proposed, taking into account the synergetic effect between Sb2O3 with POPD.

Keywords:
Antimony oxide
POPD
Ibuprofen
Photocatalytic degradation
Solarlight
Full Text
1Introduction

For the past few decades, the demand for the removal of pharmaceutically active compounds from the water bodies and water effluents has been highly studied [1,2]. These compounds enter into the water effluents due to industrial, hospitals wastewaters and secretion of the non-metabolized drug by humans and animal urine and faeces [3,4]. Among those pharmaceutical compounds, the most common are the antibiotics and non-steroidal anti-inflammatory drugs (NSAID). Ibuprofen (IB), 2-[4-(2-methylpropyl) phenyl] propanoic acid is a non-steroidal anti-inflammatory (NSAID) drug belonging to the group of propanoic acid derivatives [5,6]. It is most widely used in human health issues for the treatment of migraine, muscle aches, arthritis, tooth aches, and fever relieve [1,2,7]. The IB enters into the water bodies through faeces and urine as non-metabolized parent compound and produces serious human health issues and imbalance in the ecosystem. Thereby the development of methods to control this type of pollution is a great challenge. Various methods have been reported to eliminate such water pollutants from waste waters such as absorption, biological degradation, and photocatalytic degradation. Several studies have shown that IB cannot be completely removed by adsorption and biological treatments and the byproducts of IB can cause even more serious environmental problems [8,9].

Heterogeneous photocatalysis has been regarded as a promising technology for water remediation due to its high efficiency, low cost, easy operation, the absence of production of harmful byproducts [10,11]. The development of visible-light-driven photocatalysis has therefore received significant interest particularly due to its utilization of major portion of visible light in the solar spectrum [12].

Visible light driven photocatalysis is regarded as a “green approach” for the degradation of toxic pollutants because solar light constitutes 50% of the visible light. As metal oxides have a wide band gap, significant strategies have been adopted to improve their photocatalytic performance by coupling them with narrow band gap materials [10]. This modification can be realized by doping/coupling UV-active photocatalysts with narrow band gap materials such as metals, metal oxides and conducting polymers [13–16].

Antimony oxide (Sb2O3) with a wide band gap of 3.4 eV has been extensively used as a photocatalyst [17], flame retardant material [18], gas and chemical sensor [19–21] and in optoelectronic and photoelectric devices [22,23]. Inspite of being utilized as a photocatalyst, Sb2O3 has many disadvantages such as insufficient absorption of visible light, low surface area and fast recombination of photo-generated electron-hole pairs etc. [24]. Many methods have been adopted to solve these issues and enhance the photocatalytic activity of Sb2O3. Sb2O3 nanoparticles have been reported to be coupled with TiO2[25], ZnO [26], WO3[27], Nb2O5[28], SnO2[29], ZnBiSbO4[30], SrZrO3[31], PbO [32] and polyaniline (PANI) [33] to obtain the desired visible-light photocatalytic activity with minimum recombination of photo-generated electron-hole pairs. Conducting polymers have been used to modify the band gap energy of various metal oxide and improve the photocatalytic performance under visible light [10,34]. In our previous studies we have prepared polyl(1‐naphthylamine) PNA/TiO2[35], polypyrrole PPY/AgFeO2[36], polycarbazole PCz/TiO2[37], PNA/ZnO [38] nanohybrids and found that the hybrids exhibited enhanced photocatalytic activity under visible light [39,40]. Ppy/V2O5 and Ppy/K-Bi nanohybrids prepared via ‘in-situ’ oxidative polymerization method revealed excellent photocatalytic performance due to the sensitizing effect of conducting polymer, and the synergetic effect between conducting polymer and the metal oxide [10,11].

With a view to improve the visible light photocatalytic activity of Sb2O3, poly (o-phenylenediamine) (POPD) was chosen as it is found to be environmentally stable, has a narrow band gap of 1.5 eV and shows remarkable optoelectronic as well as electrochemical characteristics [41,42]. The polymer can be easily synthesized by chemical as well as electrochemical methods under ambient conditions. To the best of our knowledge, there are no reports in the literature on the coupling of POPD with Sb2O3. The synthesized organic-inorganic hybrids were characterized by thermogravimetric analysis (TGA), FE-SEM coupled with EDX, Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS). The results indicated that POPD/Sb2O3nanohybrids exhibited good photo-catalytic activity. The degradation fragments were identified using LCMS technique and a mechanism for the photocatalytic degradation was proposed.

2Experimental2.1Materials

o-phenylenediamine (Sigma–Aldrich, USA), Sb2O3 (Sigma–Aldrich, USA), SbCl3 (Sigma–Aldrich, USA), ferric chloride (Merck India), Chloroform (Sigma–Aldrich, USA), Ibuprofen (Cipla, USA), Ethanol (Sigma–Aldrich, USA) and deionized water were used without further purification.

2.2Synthesis of antimony oxide (Sb2O3) nanoparticles

SbCl3 (1 g) was dissolved in an acid solution (100 ml, 1 M HCl). Polyvinylalcohol (PVA) (3 g) was added to the above dispersion and the reaction mixture was sonicated for 30 min at 50 °C, followed by addition of NaOH (5%, 15 ml) drop-wise to the above mixture. The solution was further sonicated for 4 h at 30 °C, filtered and dried in vacuum oven at 80 °C. The final product was calcined at 400 °C in a muffle furnace for 2 h.

2.3Synthesis of poly o-phenylenediamine

O-phenylenediamine monomer (1.081 g, 1.6 × 10−2 mol) was added to a 100 ml conical flask containing 30 ml of deionized water followed by the addition of ferric chloride (2.59 g, 1.6 × 10−2 mol) as the initiator to the above reaction mixture. The reaction mixture was then sonicated for 4 h at room temperature. The obtained precipitate was washed with water several times and dried in the oven at 60 °C.

2.4Synthesis of POPD (poly o-phenylenediamine)/Sb2O3nanohybrids

Monomer o-phenylenediamine (1.0814, 1.0 × 10−2 mol) was dissolved in acidic solution (25 ml, 1 M HCl) followed by the addition of Sb2O3 solution (2.91 g, 1.0 × 10−2 mol) in a100 ml conical flask. The above solution was sonicated for 15 min to ensure uniform mixing and the solution was labeled A. FeCl3.6H2O (1.611, 1.0 × 10−2 mol) was dissolved in acidic solution (25 ml of 1 M HCl), sonicated for 15 min and this solution was labeled B. The solution B was transferred into solution A and the reaction mixture was sonicated for 6 h at 25 °C on an ultrasonic bath. The obtained dark reddish brown precipitate of POPD/Sb2O3 was filtered, washed several times with distilled water to remove unreacted oxidant, monomer and other impurities and dried in vacuum oven for 24 h at 70 °C. The amount of o-phenylenediamine monomer was varied and the nanohybrids were designated as 6%-POPD/Sb2O3, 12%-POPD/Sb2O3,18%-POPD/Sb2O3and 24%-POPD/Sb2O3 respectively based on the FE-SEM and TGA results that confirmed the loading of POPD in Sb2O3.

2.5Photocatalytic activity

The photocatalytic experiment was performed under solar light on an ultrasonic bath (model Soner 220 H, 53 kHz, 500 w, M/S Scope Enterprises, India). A stock solution of 50 ppm IB drug solution was prepared by dissolving 50 mg of ibuprofen in 1 L of deionized water. An aqueous suspension of IB (100 ml, 50 mg/L) was placed in a 250 ml beaker, and 50 mg of Sb2O3, POPD, and POPD/Sb2O3 nanohybrids were added. The solution was sonicated for 10 min and kept under dark condition. The solution was sonicated and exposed to solar light irradiation simultaneously for a period of 60 min. For the degradation analysis, aliquots (5 ml) of IB solution were taken out at regular intervals (0 min, 10 min, 20 min, 30 min, 40 min, 50 min and 60 min) and analyzed on a Perkin-Elmer Lambda 35 UV–Vis spectrophotometer (λmax IB = 224 nm). A calibration plot based on Beer-Lambert‘s law was obtained by plotting the absorbance against the concentration of drug in solution to determine the quantity of the drug degraded. For kinetics study, the degradation data was plotted in Origin 12 pro software.

3Characterization

FT-IR spectra of the nanohybrids and the pristine compounds were recorded on FT-IR spectrophotometer model Shimadzu IR Affinity -1 and the UV–Vis diffuse reflectance spectra were recorded on Perkin-Elmer lambda-35 UV–Vis-IR spectrometer as per reported method [10]. X-ray diffraction patterns were recorded on Philips Pw 3710 powder diffractometer (Ni-filtered Cu- Kα radiations). Field emission-scanning electron microscopy (FE-SEM) was carried out using Leo Supra 50 V P, Carl Zeiss, Germany equipped with an energy-dispersive X-ray (EDX) system. High-resolution transmission electron microscope (HR-TEM) TECNAI 200 Kv TEM (Fei, Electron Optics), the USA as per reported method [10]. The thermal stability of Sb2O3 and POPD/ Sb2O3nanohybrids was investigated using thermal analyzer STA 6000, Perkin Elmer as per reported method [10]. Liquid chromatography–mass spectroscopy (LC–MS) was conducted as per reported method [10].

4Radical scavenger studies

In order to investigate the active species generated during the POPD/Sb2O3 nanohybrid photocatalytic degradation process of IB drug as per the reported method [10]. Disodium EDTA, benzoquinone (BQ), tertiary butyl alcohol (TBA) and sodium sulfate (Na2SO4) were used as scavengers in the photocatalytic degradation process to trap the h+, O2−, OH, and e—respectively. For the radical generation experiment, 5 mg of the nanohybrid was added to 50 ml of drug solution (IB, 50 ppm) with scavenger (5 ml, 5 mM) and was irradiated under solar light. The suspension was taken at fixed intervals (0 min, 10 min, 20 min, 30 min, 40 min, 50 min, and 60 min) and analyzed spectrophotometrically.

5Result and discussion5.1Confirmation of loading of POPD in POPD/Sb2O3 via TGA, FE-SEM, XPS and XRD analyses

The TGA profile of Sb2O3, Fig. 1 (a), showed a two-step degradation process. The first step degradation revealed a mass loss of 14.7% between 68 °C to 498 °C due to the removal of water molecules and moisture. The second weight loss of 51.96% between 499 °C to 635 °C occurred due to the evaporation of Sb2O3 and oxidation to Sb2O4. The DTG curves showed main two exothermic peaks in this region, while DTA profile of Sb2O3 revealed two endothermic peaks, one broad hump which was related to the loss of water molecules. The second sharp peak was associated with the evaporation of Sb2O3 and oxidation to Sb2O4[43]. The TGA-DTA-DTG of POPD, Fig. 1 (b), revealed three major weight loss steps. The first weight loss of 18% at 103 °C was due to the expulsion of moisture while the second weight loss of 34.14% between 103 °C to 334 °C was due to decomposition of impurities and oligomers. The third major weight loss of 60% occurred between 334 °C to 700 °C due to the degradation of the POPD backbone [44]. The DTA and DTG curves of POPD, also showed three exothermic peaks. The TGA profile of 6%-POPD/Sb2O3, Fig. 1(c), revealed a four step degradation pattern. Around 6% wt. loss upto 432 °C was associated with the removal of water molecules and moisture while 17% wt. loss at 485 °C was correlated to the degradation of oligomer and decomposition of the polymer chains. The third weight loss of around 47% at 587 °C was attributed to complete degradation of skeletal POPD chain structure while the fourth weight loss of 53% between 587 °C to 700 °C was noticed due to the oxidation of Sb2O3 to Sb2O4. The DTA profile of 6%-POPD/Sb2O3 revealed four sharp endothermic peaks around 61 °C, 231.40 °C, 478 °C and 642 °C. The loading of POPD was calculated to be 6%. Similarly, TGA profile of 12%-POPD/Sb2O3,Fig.1 (d), revealed 13.3% weight loss around at 218 °C due to the removal of water molecules, 21% weight loss around at 377 °C and 53 wt.% weight loss between 377 °C to 593 °C. Almost 58.86 wt.% weight loss was noticed between 593 °C to 700 °C and the loading of POPD was calculated to be 12% in this case. Similarly, 18%-POPD/Sb2O3 and 24%-POPD/Sb2O3 revealed four step weight loss patterns and four exothermic as well as endothermic peaks were noticed in the DTG and DTA profiles. The loading of POPD was calculated to be 18% and 24% for 18%-POPD/Sb2O3 and 24%-POPD/Sb2O3 nanohybrids respectively. The thermal stability was found to in the order Sb2O3 < 6%-POPD/Sb2O3 < 12%-POPD/Sb2O3 < 18%-POPD/Sb2O3 < 24%-POPD/Sb2O3.

Fig. 1.

TGA-DTA-DTG curves of (a) Sb2O3, (b) POPD, (c) 6%-POPD/ Sb2O3, (d) 12%-POPD/, Sb2O3 (e) 18%-POPD/Sb2O3, (f) 24%-POPD/ Sb2O3 nanohybrids.

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The FE-SEM of pure Sb2O3, Fig. 2 (a) revealed the formation of cube-like structures which appeared to be highly crystalline. The energy dispersive X-ray (EDX) spectrum showed 84.26 wt.% and 15.74 wt. % of Sb and O, respectively, which was consistent with the optimal stoichiometry of Sb2O3. Peaks associated with impurities or contamination were not detected which asserted the purity of Sb2O3, The FE-SEM micrograph of POPD, Fig. 2(b), exhibited a star like morphology of elongated rod like structures of POPD while the FE-SEM of 6%-POPD/Sb2O3Fig. 2(c) showed a mixed morphology of elongated hexagonal rod like structures of POPD surrounded by cubes of Sb2O3. The EDX spectrum confirmed the loading of POPD and Sb2O3 which was found to be 6.58 wt.% and 77.07 wt.%, respectively. The FE-SEM of 12%-POPD/Sb2O3 nanohybrid, Fig. 2(d), exhibited cluster like morphology and from the EDX spectrum the percent loading of POPD and Sb2O3 in nanohybrid were calculated to be 12 wt.% and 88 wt.%, respectively. In case of 18%-POPD/Sb2O3, Fig. 2 (e), rod-like structures were seen, and the loading of POPD, in this case, was found to be 18 wt.%. The FE-SEM of 24%-POPD/Sb2O3,Fig. 2 (f), also exhibited rod-shaped morphology similar to be previous nanohybrids in which Sb2O3 cubic particles were scattered around the rods of POPD. The loading of POPD and Sb2O3 was computed to be 24 wt.% and 76 wt.%, respectively. The elemental mapping of this composition Fig. (g), revealed that the Sb2O3 particles were found to be uniformly dispersed along with POPD and formed a well-connected network like structure. The FE-SEM of the nanohybrid after degradation (which is discussed in the later section), Fig. 2(h), showed that the Sb2O3 particles encapsulated the POPD rods. The elemental mapping Fig.2 (i), also confirmed this observation and the entire surface of the nanohybrid was found to be covered with Sb2O3 while POPD was noticed to be embedded inside. The FE-SEM-EDX studies, therefore, confirmed the formation of a homogeneously dispersed POPD/Sb2O3 nanohybrid which was found to undergo encapsulation by Sb2O3.after photocatalysis.

Fig. 2.

FE-SEM with EDX of (a) Sb2O3, (b) POPD, (c) 6%-POPD/ Sb2O3, (d) 12%-POPD/ Sb2O3, (e) 18%-POPD/Sb2O3, (f) 24%-POPD/Sb2O3 nanohybrids (g) elemental mapping of 24%-POPD/Sb2O3 (h) EDX of 24%-POPD/Sb2O3 after degradation (i) elemental mapping of 24%-POPD/Sb2O3 after degradation.

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The XPS survey spectrum of Sb2O3 revealed peaks, Fig. 3(a), associated with Sb around 539.82 eV and 530.31 eV associated with Sb 3d3/2 and Sb 3d5/2. The peak associated with O 1s was noticed at 531.24 eV, Fig.3(b). The survey spectrum of 24%-POPD/Sb2O3 showed slight shifting of peaks associated with Sb 3d and O 1s, Fig.3(a). The peak correlated to Sb 3d3/2 was observed at 538.72 eV and 540.79 eV while the peak of Sb 3d5/2 was found at 529.21 eV. The peak of O 1s was noticed at 531.57 eV. The peaks were in good agreement with the reported values [45]. The shifting as of Sb peaks well as the presence of multiple Sb peaks was related to the intense interaction of Sb with POPD upon formation of organic-inorganic hybrid. The N 1s spectrum of 24%- POPD/Sb2O3, Fig. 3(c), showed peaks at 397.88 eV and 398.41 eV assigned to imine (N) while the peak at 399.52 eV was associated with amine (NH). The presence of imine peaks confirmed the conducting state of POPD [46]. The C 1s spectrum of 24%-POPD/Sb2O3, Fig. 3(c), showed peaks at 283.48 eV, 284.53 eV and 284.98 eV which are generally assigned to C-Sb, C-C/ C-H, C-O-Sb interactions [47]. The presence of peaks associated with Sb-C and C-O-Sb confirmed strong interaction between Sb2O3 and POPD.

Fig. 3.

(a) XPS survey spectra of Sb2O3 and 24%-POPD/ Sb2O3, (b) Sb 3d spectra Sb2O3 and 24%-POPD/ Sb2O3 (c) C1s and N 1s spectra of 24%-POPD/ Sb2O3 nanohybrids.

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The XRD profile of Sb2O3,Fig. 4, revealed peaks at 2θ = 13.70°, 14.56°, 22.53°, 23.59°, 27.66°, 28.46°, 32°, 35.03°, 42.05°, 44.30°, 45.95°, 54.48°, 57.12°, 59.05°, 64.02°, 67.62°, 68.80°, 73.98°, 76.25°, 76.25°, 77.93° which corresponded to the presence of (111), (110), (202), (131), (222), (400), (331), (422), (611), (440), (622), (444), (661), (731), (800), (733), (662), (840) and (911) planes. The peaks were found to match with the cubic Sb2O3 senarmontite showing cell constant as a = b = c = 11.151 A° and α = β = λ = 90° which was found to be in accordance with the JCPDF 43-1456. The XRD profile of f POPD revealed peaks at 2θ = 10.1°, 16.7°, 18.2° and 26.6° which was consistent with amorphous nature of carbon (JCPDS File no: # 00-0047). The nanohybrid revealed a slight shift in the crystalline peaks as well as variation in their intensities upon formation of nanohybrid. The nanohybrid 6%-POPD/Sb2O3 revealed peaks at 2θ = 14.32°, 18.5°, 21.86°, 24.04°, 27.18°, 28.54°, 31.34°, 31.86°, 33.82°, 34.98°, 37.64°, 42.22°, 44.18°, 46.02° which were similar to the peaks noticed in Sb2O3 while a new peak was observed at 27° due to the presence of the POPD. The intensity of this peak was observed to be highest for 18%-POPD/Sb2O3 but was found undergo reduction in case of 24%-POPD/Sb2O3. This behavior was correlated to reorganization of POPD chains upon higher loading which undergo encapsulation by Sb2O3 particles due to intense interaction which results in the intensity reduction. This has been previously observed n the Fe-SEM of the nanohybrids. The presence of the peaks corresponding to both POPD and Sb2O3 confirmed the formation of the nanohybrid. It was noticed that after photocatalysis, the nanohybrid showed insignificant changes in the XRD profile which confirmed that the planes corresponding to POPD as well as Sb2O3 remained intact and did not reveal any major structural changes reflecting the stability of the nanohybrid.

Fig. 4.

XRD of Sb2O3, POPD and POPD/ Sb2O3 nanohybrids.

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5.2Morphological analysis of POPD/Sb2O3

The HR-TEM of Sb2O3, Fig. 5(a), showed the formation of irregular cubic particles. Cubic particles fused with each other which exhibited agglomeration of dense cubic particles ranging between 10 nm–50 nm. The diffraction pattern of Sb2O3 (inset) revealed the presence of crystalline phase exhibiting lattice fringes at 1.76 Å which were in correlation with the XRD results. The HR-TEM of 24%-POPD/Sb2O3, Fig.5(b), showed dispersion of clusters of spherical particles ranging between 3 nm–50 nm and the diffraction pattern (inset) revealed lattice fringes at 0.39 Å and 0 .75 Å which were also in good agreement with the XRD results.

Fig. 5.

HR-TEM of (a) Sb2O3, (b) 24%-POPD/Sb2O3 (diffraction pattern shown in inset).

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5.3FT-IR analysis

The FT-IR spectra of Sb2O3, Sb2O5, POPD, and POPD/Sb2O3 is provided in supplementary information as Fig. S1 (a–f) while the absorption band values and their assignments are summarized in Table 1. The FT-IR spectrum of Sb2O3, showed peaks at 3737.82 cm−1, 3619.81 cm−1, 3506.29 cm−1 and 3294.65 cm−1 corresponding to OH stretching vibrations of water molecules present on the surface of Sb2O3 while the peaks at 1690.85 cm−1 and 1615.88 cm−1 were observed due to the bending vibration of OH groups of water molecules. The region spanning between 500-1100 cm−1 depicted the lattice vibrations of Sb2O3. Peaks were noticed at 1134.74 cm−1, 1054.61 cm−1, 829.44 cm−1 and 719.02 cm−1 corresponding to anti-symmetric and symmetric stretching vibrations of Sb-O-Sb and O-Sb-O, respectively [48].

Table 1.

IR absorption bands of POPD/Sb2O3 nanohybrids.

Sample  Functional group  Wavenumber (cm−1
Sb2O3Group of water molecules (OH stretching and bending vibrations)  3737.823619.813506.293294.651690.95 1615.88 
Sb-O-Sb symmetric and antisymmetric stretching vibrations  1134.741054.61 824.44 719.02 
POPDPOPD molecules (NH stretching of NH3140.38 
POPD molecules (NH stretching of NH23289.81 
POPD molecule(CC streching vibration of quionoid)  1684.14 
POPD molecules (CC stretching vibration of benzoin ring)  1524.44 
POPD molecule (CN streching vibration)  1631.70 
POPD molecule (CN streching vibration)  1362.321233.44 
POPD molecule (CH out of plane)  832.94742.21 
6%-POPD/Sb2O3Group of water molecules (OH streching and bending vibrations)  3737.82 3619.813506.29 3294.65 1690.951615.88 
POPD molecules (NH stretching of NH3127.03 
POPD molecules (NH stretching of NH23292.56 
POPD molecule (CC stretching vibration of benzoin ring)  1527.63, 1688.61 
POPD molecule (CN streching vibration)  1630.49 
POPD molecule (CN streching vibration)  1364.38, 1235.25 
Sb-O-Sb symmetric and antisymmetric stretching vibrations  1143.05, 711.73 
12%-POPD/Sb2O3Group of water molecules (OH stretching and bending vibrations)  3731.94 3604.39 
POPD molecules (NH stretching of NH3289.81 
POPD molecules (NH stretching of NH23097.94 
POPD molecule (CN streching vibration)  1375.31 1242.221209.60 
POPD molecule (CC stretching vibration of benzoin ring)  1613.091532.80 
POPD molecule (CH out of plane)  832.11 
Sb-O-Sb symmetric and antisymmetric stretching vibrations  1147.551034.34 715.34 
18%-POPD/Sb2O3Group of water molecules (OH stretching and bending vibrations)  3733.913614.14 
POPD molecules (NH stretching of NH3115.54 
POPD molecules (NH stretching of NH23289.81 
POPD molecule(CC streching vibration of quionoid)  1688.76 
POPD molecules (CC stretching vibration of benzoin ring)  1531.12 
POPD molecule (CN streching vibration)  1632.49 
POPD molecule (CN streching vibration)  1364.521211.10 
Sb-O-Sb symmetric and antisymmetric stretching vibrations  1145.54717.49 
POPD molecule (CH out of plane)  832.97 
24%-POPD/Sb2O3Group of water molecules (OH streching and bending vibrations)  3732.443610.54 
POPD molecules (NH streching of NH3038.01 
POPD molecules (NH streching of NH23124.54 
POPD molecule(CC streching vibration of quionoid)  1688.74 
POPD molecules (CC streching vibration of benzoid ring)  1402.791528.65 
POPD molecule (CN streching vibration)  1632.30 
POPD C-H bending  1216.43 
POPD molecule (CH out of plane)  835.73 
Sb-O-Sb symmetric and antisymmetric streching vibrations  1143.27 1091.49 960.96 718.77 

The FT-IR spectrum of POPD showed intense peaks at 3289.81 cm−1 and 3140.38 cm-1 which were attributed to the NH stretching vibrations of the NH and NH2 groups, respectively. The peaks at 1684.14 cm−1 and 1524.44 cm−1 corresponded to the CC stretching vibrations of quinonoid and benzenoid rings, while a strong peak at 1631.70cm−1 was associated with the imine stretching vibration. The peaks at 1362.32 cm−1and 1233.44 cm−1 corresponded to the presence of C–N stretching vibrations of o-phenylenediamine units. The characteristics peaks at 832.98 cm−1 and 742.21 cm−1 were noticed due to the 1,2,4-trisubstituted and 1,2,3-trisubstituted benzene rings. The polymerization of POPD was therefore confirmed [49–52]. The FT-IR spectrum of the POPD/Sb2O3 nanohybrids showed similar characteristic bands of as that of POPD and Sb2O3. The intensity of the peaks was found to increase with the increase in the amount of POPD in Sb2O3which confirmed the synergistic interaction between the two moieties.

The UV-diffuse reflectance spectra of Sb2O3, POPD and POPD/ Sb2O3 nanohybrids are depicted in Fig. 6 (a). All the nanohybrids revealed a decrease in the percent transmittance as compared to that of pure Sb2O3. The transmittance value was observed to be 70% for 6%-POPD/Sb2O3 while the transmittance value was found to be 50% for 12%-POPD/ Sb2O3. The nanohybrid 18%-POPD/Sb2O3 revealed transmittance value of 40%, while the value was noticed to be 20% for 24%-POPD/Sb2O3

Fig. 6.

(a) Diffuse reflectance spectra of Sb2O3, POPD and POPD/Sb2O3 nanohybrids, (b) plot of (αhν)1/2 versus photon energy of POPD/Sb2O3 nanohybrids (inset Sb2O3).

(0.37MB).

The optical band gap (Eg) was calculated using the equation:

α(hv) = A(hv- Eg)n …………………………….(4.1)

Where A is the constant, v is the frequency of radiation, Eg is the band gap, h is the plank constant, α is absorption coefficient and it can be calculated as α = 2.303 log(T/d) ( where T is the transmittance and d is the thickness of sample) or α = 2.303(A/d) ( where A is the absorbance) and hv can be calculated from wavelength : hv = 1240/wavelength, n = ½ for the allowed direct bandgap whereas n = 2 for the indirect band gap. Plot (αhν )1/2 as a function of hν then we have the sample with an indirect band gap. Plot of (αhν)1/2 vs. energy (eV), Fig. 6(b), revealed the optical band gap values as 3.35 eV, 1.5 eV, 1.6 eV, 1.45 eV, 1.4 eV and 1.35 eV for Sb2O3, POPD, 6%-POPD/Sb2O3, 12%-POPD/Sb2O3, 18%-POPD/Sb2O3 and 24%-POPD/Sb2O3 respectively. Thus the results showed that the band gap was remarkably improved upon the addition of POPD. The formation of nanohybrids revealed a reduction in the band gap of Sb2O3. Therefore, the POPD/ Sb2O3 nanohybrids could be efficiently used to produce more electron-hole pairs under visible light illumination.

5.4Photocatalytic degradation studies of Ibuprofen under solar light irradiation

The degradation of IB drug was performed in presence of pure Sb2O3, POPD, and POPD/Sb2O3 nanohybrids as catalysts under solar light. Fig. 7(a), shows the degradation spectra of the drug in the presence of pure of pure Sb2O3, POPD and POPD/Sb2O3 nanohybrids in presence of solar light. The degradation efficiency of the IB drug using Sb2O3, POPD and POPD/ Sb2O3 nanohybrids was observe to be 51.13%, 80.90%, 64.88%, 85.70%, 87.40% and 91.40%, respectively, under solar light irradiation for 60 min. Pristine POPD and POPD/Sb2O3 nanohybrids showed higher degradation as compared to Sb2O3. It was noticed that with the increase in the amount of the POPD in Sb2O3 the photocatalytic efficiency was increased. The degradation was increased to almost 99% upon increasing the loading of POPD to 30% (given in supporting in formation as Fig S2). The plots of 1/C versus time Fig. 7(b) showed that degradation kinetics followed the second order for Sb2O3, POPD, and POPD/Sb2O3 nanohybrids. The rate constant was calculated to be 0.013 and 0.064 for Sb2O3 and POPD, respectively. In case of POPD/Sb2O3 nanohybrids, the rate constant values were observed to be 0.031, 0.09, 0.12 and 0.17 for 6%-POPD/Sb2O3, 12%-POPD/Sb2O3, 18%-POPD/Sb2O3 and 24%-POPD/Sb2O3, Table 2.

Fig. 7.

(a) Percent degradation of IB in presence of Sb2O3, POPD, and POPD/ Sb2O3 nanohybrids under solar light, (b) 1/C versus time plot for IB drug in presence of Sb2O3, POPD, and POPD/ Sb2O3 nanohybrids.

(0.25MB).
Table 2.

Apparent second-order rate constants (k) of IB degradation and linear regression coefficients from a plot of 1/C versus time.

Photocatalysts  Rate constant (K)(Under Sunlight)  R2 (Sunlight ) 
Sb2O3  0.0126  0.9689 
POPD  0.0642  0.9751 
6%-POPD/Sb2O3  0.0309  0.9923 
12%-POPD/Sb2O3  0.0999  0.9936 
18%-POPD/Sb2O3  0.1216  0.9827 
24%-POPD/Sb2O3  0.1725  0.9814 

The recyclability of 24%-POPD/Sb2O3 was tested up to five cycles and in each cycle the photocatalyst was irradiated for a period of 60 min. The nanohybrid revealed 86.06% degradation i.e. hardly 5.34% loss of catalytic activity after five cycles which showed that it exhibited good photocatalytic stability and maintained its photocatalytic activity even after five cycles, Fig. 8(a).

Fig. 8.

(a) Recycle ability test of 24%-POPD/Sb2O3 up to 5 cycles (b) radical generation plot.

(0.92MB).

To explore the photocatalytic mechanism of IB degradation in presence of the nanohybrids, radical scavenging studies were carried out using p-benzoquinone (BZQ, O2· radical scavenger), disodium ethylene diamine tetraacetate (Na2-EDTA, hole scavenger h+), tert-butanol (t-BuOH, OH· radical scavenger), sodium sulfate (Na2SO4, electron scavenger e), Fig. 8(b). The rate of degradation was observed to be 90.86%, 90.64%, 82.80% and 82.40% in presence of Na2-EDTA, Na2SO4, BQ and TB, respectively. The result strongly indicated that the OH and O2 radicals were the active species involved in photocatalytic degradation of IB. The addition of Na2-EDTA, sodium sulfate showed no significant effect on the photocatalytic activity of POPD/Sb2O3 while the presence of t-BuOH and BQ decelerated the photocatalytic degradation of IB. Therefore, it can be concluded that OH and O2 radicals were the dominant species responsible for the degradation if IB drug using POPD/Sb2O3.

5.5Mechanism of the POPD/Sb2O3 nanohybrids

Sb2O3 nanoparticles when irradiated with solar light generate electron-hole pairs, which can react with water to yield hydroxyl and superoxide radicals. These radicals oxidize and mineralize the drug molecules. However, the band gap of Sb2O3 is 3.35 eV, which means that only UV light can excite the Sb2O3 nanoparticles to generate electron-hole pairs. POPD has a band gap of 1.57 eV, which is narrower than that of Sb2O3 (3.35 eV), showing strong absorption in the region of visible light.

Therefore, POPD can act as a photosensitizer for Sb2O3. When POPD/Sb2O3 nanohybrid is illuminated with visible light both Sb2O3 and POPD absorb photons at their interface because the CB of Sb2O3 and the LUMO of POPD are well matched for charge transfer. Electrons generated by POPD are transferred to the CB of Sb2O3, enhancing charge separation and promoting the photocatalytic ability, Scheme 1. The free electrons react with O2 to produce superoxide radical (O2), and the holes (h+) react with OH and H2O to produce a hydroxyl radical (OH). POPD/Sb2O3 hinders the recombination of the hole and electron and facilitates the generation of ROS that degrades IB. Therefore, enhanced photocatalytic activity mechanism is based on synergetic effect between POPD and Sb2O3.A comparative performance of various Sb2O3 reported by other authors Table 3 clearly reveals that the degradation efficiency of POPD/Sb2O3 nanohybrid was observed to be superior to the reported Sb2O3 photocatalysts.

Scheme 1.

Mechanism of photocatalytic action of POPD/Sb2O3 nanohybrid under solar light.

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Table 3.

Comparison of reported photocatalytic efficiencies of Sb2O3.

Photocatalyst  Pollutant  Light irradiation  Degradation time  Degradation (%) 
Sb2O3[53]  Acridine orange (AO)  UV–vis  150 min  63 
S-Sb2O3[17]  Methyl orange (MO) and 4-phenylazophenol  Visible  1 h ( MO) 150 min (4-phenylazophenol)  99.2 using S- Sb2O3 at pH = 12 for MO94.3 using S- Sb2O3 at pH = 12 for 4-phenylazophenol 
Sb2O3/TiO2[25]  Methylene blue (MB), methyl orange (MO)  Visible  60 min  82. 42 (0.5% Sb2O3/TiO2
Sb2O3- ZnO [26]  Methylene blue (MB)  UV  90 min  71 
Nb2O5/Sb2O3[28]  Orange G dye  UV–vis  120 min  86.74 
ZnBiSbO4[30]  Indigo carmine (IC)  Visible  270 min  100 
SrZrO3-Sb2O3[31]  tetracycline  UV/ UV–vis  3h  70 
Sb2O3/WO3[27]  Rhodamine B  Visible   4h   
Sb2O3/PbO[32]  Carbamazepine (CBZ )  UV  120 min  96.34 using Sb2O3/PbO (at pH = 2) 
POPD/TiO2[49]  Methylene blue (MB)  Visible  3 h  91.30 
MVO4/PoPD(M = Bi/La)[51]  methylene blue (MB)  Visible light  120 min  96.30 and 70.40 using BiVO4/PoPD and LaVO4/PoPD, respectively 
PMPD/ZnO [52]  CI Acid Red 249  UV/Visible  2 h  94 
5.6Proposed degradation pathway of IB drug

Photocatalytic degradation of Ibuprofen drug was confirmed by the LCMS analysis which revealed that the drug was successively degraded into intermediate compounds of decreasing m/z values, Scheme 2 (LCMS profile given in supporting information as Fig. S3). The intermediate with 100% abundance was takes as the main degraded product of drug and the other intermediates compounds were labeled as P1 to P20. The first intermediate P1 (m/z 192) was obtained due to loss of methyl group and the second intermediate P2 (m/z 225) showed the 100% abundance and was taken as the main degradation product. Intermediates P3, P4 with m/z values 222 (30%), 238 (15%) were obtained from the parent drug molecules through the OH radical attack. The fragment degraded into P4, (m/z 178) [1,4 isophenyl propanol] via elimination of carboxylic acid group and converted in to P5 fragment with m/z value 160. Other intermediates with m/z values P7-166, P8-238, P14-110, P15-126, P17-84, P18-82 and P19-154 were obtained. Fragment P7, (m/z 166) [2-(4- hydroxyphenyl)-propionic acid] was obtained from P3, (m/z 178) via the elimination of isopropyl alcohol and also by attack of OH radical. P7 fragment degraded into P8, (m/z 138) [2-(4- hydroxyphenyl)-propanol] due to cleavage of the carboxylic acid group followed by attack of OH radical. The fragment degraded into P14, (m/z 110) [1,4-hydroquinone] by elimination of ethyl alcohol group which on further by attack of OH experienced fission of the benzene ring to generate fragment P15 [2 hydroxy-1, 4 quinol] with m/z- 126, P17 [3-methylbuta-1,3 dien-2- ol] with m/z- 84 and P18 [1,3 butadien-2,4 diol) ]with m/z -82. The degradation product P19 was formed by fission of the ring followed attack of O2– radical. The intermediates from P12 (m/z 132), P13 (m/z 150) and P16 (166) were obtained from P9 [isophenylacetophenone] with m/z 176 via elimination of isopropyl group and acyl group, and also by attack of OH radicals.

Scheme 2.

Degradation pathway of Ibuprofen drug by LCMS.

(0.52MB).
6Conclusion

POPD/Sb2O3 nanohybrids were successfully synthesized via an in-situ oxidative polymerization method. The modified photocatalysts were characterized by XRD, FE-SEM-EDX, TEM, FT-IR, TGA, and UV–Vis DRS. The loading of POPD in Sb2O3 was confirmed by TGA and FE-SEM-EDX analysis. IR analysis showed the formation of nanohybrids while XRD results confirmed crystalline morphology of the nanohybrids. The optical band gap was found to be 1.35 eV with 24% loading of POPD in Sb2O3. The photocatalytic degradation of (IB) showed almost 91.49% degradation of 50 ppm IB solution in 60 min under solar light irradiation. The results indicated that POPD/Sb2O3 nanohybrids exhibited good photocatalytic activity and radical scavenger experiments validated that the photogenerated OH and O2– radicals were the two main photoactive species responsible for photocatalytic degradation. The possible mechanism of photocatalytic activity and degradation of drug into small fragments was proposed. The LCMS analysis revealed the formation of low molar mass fragments. Thus, the POPD/Sb2O3 nanohybrids could be used as effective photocatalysts for the degradation of toxic organic pollutants.

Conflicts of interest

The authors declare no conflicts of interest

Appendix A
Supplementary data

The following are Supplementary data to this article:

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